[clasim]

Are you blowing air into the overflow connection ?

If you're using a cap w/ a dangle vent then your results could be normal, a cap w/a spring loaded vent will seal tight every time.

If coolant is blowing out the overflow before the engine warms up and you're using a cap w/a spring vent, you've blown a head gasket - that's the only way enough pressure could build inside the cooling system that quickly. If you're using a dangle vent cap, you could blow coolant if the vent is stuck open, replace it.

If you're loosing coolant you've got either an external or an internal leak, hopeing for the former.